Max Harris wrote:
>
>      Can anyone out there help me with the subject of using guard rings
>      around op amp inputs to minimize leakage at the high impedance nodes?
>      I will be using an FR-4 PCB, multilayer, PTH technology. From what
>      I've read, it is suggested that the guard rings be placed on every
>      layer.

        The nature of your question implies an analog world, where high
        impedance can mean in the meg-ohms - not the impedance range of
        the board itself.  With an input _that_ sensitive, it is advisable
        to make full use of the fact that Farraday was right.

        For the guard ring to be fully functional, you want sheilding be-
        tween your input and _any_ other possible affector.  This may in-
        clude putting a void in any plane layers, and if the op-amp is a
        through-hole part you should carry the guard to all layers.

>      This begs the question: "How do I then connect traces to the
>      node being guarded?". Also, any traces that do connect to the guarded
>      node and then run outside of the ring are now subject to the same
>      leakage that the original node was.
>
        In such an impedance range, the source of your signal had better
        be close enough to not be an issue as far as including it in your
        guard ring.  If there is a large value resistor in the circuit as
        it leads up to your input, this is the point where the impedance
        jumps, and is the point where the guard should extend to.  If not,
        then it may be such a signal back to the last device, connector,
        or summing junction.

        Length in such a signal path is your enemy.

        Good luck,
--

      Jeff Seeger                             Applied CAD Knowledge Inc
      Chief Technical Officer                      Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                    978 649 9800

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